Magnetic core structures

ABSTRACT

A three-legged magnetic core structure having a plurality of stacked layers of metallic laminations, including two outer leg laminations, an inner leg lamination, and joining upper and lower yoke laminations. Each layer of laminations is of like construction, utilizing laminations of like configuration and dimensions, with the layers being oriented differently to provide three layers between repeating joints. The joints at the outer corners of the magnetic core are mitered, and the joints between the yoke and inner leg lamination include a mitered and a square joint at each end of the inner leg lamination. The inner leg lamination has one end narrowed where it joins a yoke lamination with a square joint, to provide the required joint distribution between the inner leg lamination and adjoining yoke laminations.

United States Patent Inventor Theodore R. Speclrt Sharon, Pa. Appl. No.860,474 Filed Sept. 24, 1969 Patented Mar. 9, 1971 Assignee WestinghouseElectric Corporation Pittsburgh, Pa.

MAGNETIC CORE STRUCTURES 11 Claims, 10 Drawing Figs.

[1.8. CI 336/217 lnt.Cl H01f27/24 Field of Search 336/216, 217, 233, 234

References Cited UNITED STATES PATENTS 3,212,042 10/1965 Twomey 336/217336/217 3,214,71 10/ 1965 Graham 3,283,281 11/1966 Steinetal...3,303,448 2/1967 Farry Primary Examiner-Thomas .l. Kozma Attorneys-A. T.Stratton, F. E. Browder and Donald R.

Lackey ABSTRACT: A three-legged magnetic core structure having aplurality of stacked layers of metallic laminations, including two outerleg laminations, an inner leg lamination, and joining upper and loweryoke laminations. Each layer of laminations is of like construction,utilizing laminations of like configuration and dimensions, with thelayers being oriented differently to provide three layers betweenrepeating joints. The joints at the outer corners of the magnetic coreare mitered, and the joints between the yoke and inner leg laminationinclude a mitered and a square joint at each end of the inner leglamination. The inner leg lamination has one end narrowed where it joinsa yoke lamination with a square joint, to provide the required jointdistribution between the inner leg lamination and adjoining yokelaminations.

Patented March 9, 1971 3 Sheets-Sheet 1 FIG.|.

M Z mMv/nw- 6 INVENTOR Theodore R. Specht 7 EL/26 Zacfl/ ATTORNEYPatented March 9, 1971 3 Sheets-Sheet 2 III FIG-4.

MAGNETIC cons STRUCTURES BACKGROUND OF THE INVENTION 1. Field of theInvention The invention relates in general to electrical inductiveapparatus, such as transformers and reactors, and more specifically tomagnetic core structures for electrical inductive apparatus.

2. Description of the Prior Art Three-legged magnetic core structures ofthe stacked type conventionally utilize mitered or diagonal jointsbetween the outer leg and yoke laminations, and between the inner leglamination and adjoining yoke laminations; such as taught by [15. Pat.No. 2,300,964, which is assigned to the same assignee as the presentapplication. Further, it has been found that by staggering or steppingthe joints from layer to layer, with two or more laminations separatingrepeating joints, that the core losses may be substantially reducedcompared with i the butt-lap pattern wherein only one laminationseparatesrepeating joints. An example of stepped-lap construction isdisclosed in U.S. Pat. No. 3,153,215, which is assigned to the sameassignee as the present application.

The conventional three-legged magnetic core, in order to obtain allmitered or diagonal joints, provides two diagonal cuts on each end ofthe inner leg lamination, forming a spear or V-shaped point on each endof this lamination. The shearing of the ends of the inner leg laminationto a V-shape has the disadvantage of producing scrap, which amounts toabout 4 percent of the total core weight. lt'also has the disadvantageof complicating automated production of the laminations, as the doublecut on each end of the inner leg lamination cannot be accomplished by asingle oscillating shear, as are the cuts for the remaining laminations,requiring a separate operation for each end of the inner leg lamination.

Attempts to eliminate the V-shaped ends of the inner leg lamination, byutilizing a single diagonal cut on each end of this lamination, resultsin creating a mitered and a square joint between each end of the innerleg lamination and adjoining yoke laminations. While this structurelends itself to automatic shearing of the laminations from a strip ofmagnetic material, with negligible scrap, the square joints increase thecore losses when used in a structure wherein the inner leg lamination ismerely rotated I80 about a predetermined axis from layer'to layer,producing an X-like pattern, with one lamination between repeatingjoints in the same'plane. The X joint pattern at the inner leg, inaddition to lending itself to automatic shearing in a substantiallyscrapless manner, also increases the mechanical strength of the magneticcore and results in a more complete joint closure above and below theinner leg lamination, compared with magnetic cores which have inner legshaving the V-shaped points at the extreme ends thereof.

US. Pat. No. 3,283,281, which is assigned to the same assignee as thepresent application, discloses that magnetic cores may utilize squarejoints with negligible impairment of magnetic performance, if the numberof square joints per layer does not exceed two, and there are at leasttwo intervening layers of laminations before the joint repeats in-thesame plane. Pat. No. 3,283,281 discloses practical embodiments of thisconcept, utilizing at least three different punching layers in the basicpattern. The number of different lamination shapes and dimensions, andthe number of different punching layers required for the basic patterndetermines the complexity involved in manufacturing and stacking themagnetic core, and therefore has a direct influence on the manufacturingcost of the core. Thus, it would be desirable to provide a new andimproved three-legged magnetic core structure which facilitatesautomatic shearing with negligible scrap, which has mitered jointsexcept for two square joints per layer of laminations, and which has atleast two layers separating repeating joints in the same plane. Further,the manufacturing and assembly of the magnetic core should befacilitated by requiring a minimum number of different 'laminationshapes, and a minimum number of different basic punching layers.

SUMMARY OF THE INVENTION Briefly, the present invention is a new andimproved threelegged magnetic core structure of the stacked type, havinglaminations which may be cut with an oscillating shear in an automatedmanner, with substantially no scrap. Further, it has the mechanicalstrength advantages of the X joint pattern at the junction of the innerleg lamination with the adjoining yoke laminations, without the usualdisadvantages of square joints relative to magnetic performance, as ithas three layers of laminations betweenrepeating joints in the sameplane. Only one basic layer construction is used, with each layer havingseven laminations of five different configurations. Each layer oflaminations is of like construction, with the basic layer pattern beingsuch that by rotating the basic layer about three different axes, thejoints will be distributed into different planes. The requireddistribution of the joints between the inner leg lamination andadjoining yoke laminations, from layer-to-layer, is achieved bynarrowing the width of one end of the inner leg lamination, where theinner leg lamination joins a yoke lamination with a square joint.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of theinvention will become more apparent when considered in view of thefollowing detailed description and drawings, in which:

FIG. I is an elevational view of a transformer having a magnetic coreconstructed according to an embodiment of the invention;

FIG. 2 is a fragmentary view of the joint pattern between the inner legand upper yoke laminations of the magnetic core shown in FIG. 1, takenin the direction of the arrows II-II;

FIGS. 3, 4, 5 and 6 illustrate the layers of a basic group oflaminations of the'magnetic core shown in FIG. 1;

FIGS. 7 and 8 illustrate a method of cutting the leg and yokelaminations, respectively, for the magnetic core shown in FIG. I;

FIG. 9 is an elevational view of a magnetic core constructed accordingto another embodiment of the invention; and

FIG. 10 illustrates a method of cutting the leg laminations for themagnetic core shown in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings,and FIG. I in particular, there is shown a transformer 20 having athree-legged magnetic core assembly 22 constructed according to anembodiment of the invention. Magnetic core 22 includes first and secondouter leg members 24 and 26, an inner leg member 28, an upper yokemember 30, and a lower yoke member 32. Phase winding assemblies 34, 36and 38, shown in phantom, are disposed about leg members 24, 28 and 26,respectively. As illustrated, transfonner 20 is of the core-form type,with FIG. 1 being an elevational view, but by changing the relativeproportions of the leg members, magnetic core 22 may be of thesingle-phase, shell-form type, having the winding portions disposedabout the inner leg member 28. In the shell-form embodiment of theinvention, the view of the magnetic core shown in FIG. 1 would be a planview.

In general, magnetic core 22 includes a plurality of stacked layers oflaminations formed from magnetic strip material having at least onepreferred direction of magnetic orientation lengthwise of the materialor substantially parallel with the sides of the strip material. Thelaminations which form the various layers included in the magnetic corestructure 22 are assembled with their adjoining edges substantiallyaligned to form a substantially rectangular core having two rectangularwindows or openings 21 and 23 for receiving the phase windings. Eachlayer of laminations includes three-leg laminations, and yokelaminations which connect the ends of the leg laminations to form arectangular core having substantially rectangular openings. All of thejoints formed between the various yoke and leg laminations aredistributed such that when the various layers of which the magnetic core22 is formed are superposed, there are at least three laminationsseparating repeating joints in the same plane.

In order to provide a substantially scrapless three-legged magnetic corewhich lends itself to automatic shearing of the core laminations, twosquare joints per layer are utilized, which also provides advantages inthe ease of joint closure adjacent the inner leg and a strong coremechanically. The usually higher losses associated with square jointsare overcome by distributing the joints to provide three laminationlayers between repeating joints. The shearing of the lamination from astrip of magnetic material, and the stacking of laminations to form themagnetic core, are facilitated by reducing the number of differentlamination shapes to five, and by using only one basic layerconstruction, arranged to distribute the joints when the basic layer isoriented into four different positions and the layers are stacked orsuperposed. A three-legged layer of laminations has four possibleorientations, two on each side of the layer of laminations, thusproducing a magnetic core structure having a basic lamination group offour stacked layers.

More specifically, magnetic core 22 includes one or more basic groups oflayers of laminations, with each group including four layers oflaminations providing four joint planes at the outer corners of themagnetic core as illustrated in FIG. 1 by t the solid joint corner andthe three joints shown by the dotted lines at each corner, with thejoints at each corner being offset by a predetermined incrementaldimension, such as oneeighth or one-fourth of an inch, with thedimension selected usually depending upon the physical size of themagnetic core. The actual joint-to-joint overlap, however, may be morethan the selected increment, asthe joints do not necessarily stepprogressively, such as in the stepped-lap magnetic core disclosed in thehereinbefore-mentioned U.S.'Pat.

Eight different joint planes, four for the mitered joint and four forthe square joint, are provided above and below the inner leg member 28.This is essential to the good magnetic performance of the magnetic core,and is made possible by stepping or narrowing one end of each inner leglamination, where the inner leg lamination joins a yoke lamination witha square joint. In order to facilitate complete joint closure betweenthe inner leg and yoke lamination, and to provide a strong jointmechanically, the orientation of the layers of a basic group is suchthat the joint-to-joint pattern provides a succession of X joints. Thisis illustrated more clearly in FIG. 2, which is a fragmentary view ofmagnetic core 22 shown in FIG. 1, taken in the direction of arrows11-11.

The construction of magnetic core 22 may most easily be understood byexamining a basic layer of laminations, and then noting how the basiclayer is oriented to provide the other layers of a basic group. Thebasic layer is shown in FIG. 3, with the reference numeral 40, and itcorresponds to the top layer of laminations of the magnetic core 22shown in FIG. 1. Basic layer 40 includes first and second outer leglaminations 42 and 46, respectively, a center or inner leg lamination44, first and second upper yoke laminations 48 and 50, and first andsecond lower yolk laminations 52 and 54, respectively. The ends of thefirst and second outer leg laminations 42 and 46 are each cutdiagonally, preferably at an angle of substantially 45 with respect tothe direction of magnetic orientation of the strip material from whichthe laminations are cut. Each end of the inner leg lamination 44 has asingle diagonal cut, also preferably at an angle of substantially 45 tothe edges of the strip, forming edges 56 and 58, which, in thisembodiment, are parallel with one another, forming the inner leglamination into a parallelogram configuration.

In order to distribute the joints between the inner leg lamination 44and the adjoining yoke lamination into different planes, in the basicgroup of four layers of laminations, one end of the inner leg lamination44 is narrowed by a predetermined dimension, such as by one-fourth,three-eighths, or one-half of an inch, by a step which starts at one ofthe extreme ends of the lamination and extends inwardly parallel to theside or edge of the lamination, extending for at least the distance ofthe yoke width dimension. Thus, inner leg lamination 44 is narrowed by adimension K at its end adjacent the upper yoke lamination 50, but itwould be equally effective to narrow the end adjacent the lower yokelamination 54. It is important, however, that only one end be narrowed.

The ends of the first upper and first lower yoke laminations 48 and 52,respectively, are each cut diagonally with respect to the sides of thelamination, forming a substantially trapezoidal configuration, and thesecond upper and second lower yoke laminations 50 and 52, respectively,each have one end cut diagonally with respect to the sides of thelaminations, and the other ends are cut perpendicular to the sides ofthe laminations. It is important to note that the first upper and firstlower yoke laminations are of similar configuration and dimensions, andthat the second upper and second lower yoke laminations are of similarconfiguration and dimension, thus reducing the number of differentlamination shapes to five for the basic layer 40.

In the assembly of the basic layer 40 the various yoke and leglaminations are assembled to form a rectangular configuration having twosubstantially rectangular windows 21 and 23. The leg laminations aredisposed in spaced parallel relation, with the yoke laminations joiningthe ends of the leg laminations to complete the rectangularconfiguration. All of the joints between the leg and yoke laminationsare mitered, except for a square joint between the narrowed portion ofthe inner leg lamination 44 and the upperyoke lamination 50, and asquare joint between inner leg lamination 44 and the lower leglamination 54.

A rectangular three'legged layer of laminations has four possibleorientations, two on each side of the layer. All four of theseorientations are used to provide maximum joint distribution for a singlebasic layer of laminations. The mitered joints between the outer leg andyoke laminations are arranged such that as the basic layer is placedinto the possible orientations, the outer corner joints, at each outercorner of the core, will be incrementally offset into four differentplanes. The incremental narrowing of one end of the inner leg laminationaccomplishes this result for both the mitered and square joints betweenthe inner leg and adjoiningj yoke laminations. The strongest jointmechanically, and the best joint closure above and below the inner leglamination, will be obtained by disposing the mitered joints of theinner leg lamination of one layer, perpendicular to the mitered jointsof the preceding layer. In other words, the next layer of laminationsimmediately adjacent layer 40 is shown in FIG. 3, should be obtained byrotating the basic layer about axis 62, which is an axis disposedlongitudinally through the inner leg lamination 44 in the plane of thelamination, with its rotation about the axis being indicated by arrow64. Or, the basic layer 40 may be rotated 180 about axis 66, asindicated by arrow 68, which axis is perpendicular to the leglaminations in the plane of the laminations. Rotating the basic layer 40about either axis 64 or axis 68, turns the basic layer of laminationsover to its opposite side. Assuming that the second layer orientation isin rotational symmetry with the first layer about axis 62, this layerwould have the arrangement shown in FIG. 4, with the same referencenumerals being used in FIGS. 3 and 4 to indicate like laminations, withthe reference numeral for the second layer and its laminations having asingle prime mark to distinguish them from the first layer and itslaminations.

The third layer of laminations should now return the diagonal jointsabove and below the inner leg lamination to the direction of the firstlayer, but incrementally offset from the mitered joints of the firstlayer. This orientation is achieved by rotating the first layer 180about an axis 70, as indicated by arrow 72, which axis is perpendicularto the plane of the inner leg lamination 4d. The third layeroflaminations is shown in FIG. 5, and it has the same reference numeralsas the first layer, with the addition of double prime marks todistinguish the laminations from the first layer.

The fourth layer of the basic group is shown in FIG. 6, and it is inrotational symmetry with the first layer 40 about the remaining axis 66.The fourth layer has the same reference numerals as the first layer 40,with the addition of triple prime marks.

.' Thus, the second, third and fourth layers are in l80 rotationalsymmetry with the first layer 40 about axes 62, 70 and 66, respectively,but as hereinbefore pointed out, it would be equally suitable to use asequence which includes axes 68, 70 and 62, respectively, for thesecond, third and fourth layers. By using the sequence about axes 62, 70and 66, respectively, as illustrated in the FIGS., the ends of the innerleg laminations at the upper yoke appear as shown in FIG. 2, withexcellent distribution into four different planes, with the joints inthe upper yoke appearing at 80 in the first layer, 82 in the secondlayer, 84 in the third layer, and 86 in the fourth layer.

As hereinbefore stated, the joints at the four outer corners of themagnetic core are distributed into four different planes in the basicgroup of laminations. A method for choosing dimensions to stagger thejoint lines into four different planes is as follows, using the lettersshown in FIG. 3. Specifically, each window 21 and 23 has a widthdimension a, the width of the yoke lamination is z, and the widthof theleg lamination is w. The inward stepo'n the inner leg lamination is k,the shorter side of the yoke laminations 48 and S2 is n, and the shorterside of yoke'laminations 50 and 54 is n d. The dimensions S 8 S and S.are the dimensions from the start of the mitered edges of the yokelaminations, which edges cooperate with similarly cut edges of the outerleg laminations to form the outer corner joints, to the nearest side oredge of the inner leg lamination, with S S S and S being these dimensionfor yoke laminations 50, 48, 54 and 52, respectively. The followingequations may then be written for the dimensions 8,, S S and S It shouldbe noted at z w t. The values of d, k and t are selected so theright-hand sides of the four equations listed above are different. Thevalue of 2 will usually be 0, one-half, or 1 inch. Some practical goodcombinations are listed in Table 1.

TABLE I 1 1 o )6 V a a A a 0 o A 1% 1% a a a The laminations formagnetic core 22 shown in FIG. 1 may be formed in a substantiallyscrapless manner as shown in FIGS. 7 and.8. FIG. 7 illustrates thecutting of the first outer,

inner and second outer leg laminations 42, 44 and 46, respectively, froma strip 90 of magnetic material. The inward step or narrowing of one endof the inner leg lamination 44 is illustrated by the dotted line 92,with this small piece being the only scrap generated. The strip 90advances in the direction of 'arrow 94, thus facilitating the forming ofcut 92, as it will be While it is preferable to make the inner leglamination 44 in the configuration of a parallelogram, as it facilitatesthe automatic shearing of the leg laminations, the advantages of the invention may also be obtained by constructing the inner leg lamination inthe shape of a trapezoid. One end of the inner leg lamination will alsobe narrowed in this embodiment, to distribute the joints above and belowthe inner leg lamination into different planes, in a basic group oflaminations. This embodiment of the invention is shown in FIG. 9,illustrating a magnetic core 100 having a basic layer which includesfirst and second outer leg laminations 102 and 104, an inner leglamination 106, first and second upper yoke laminations 108 and 110, andfirst and second lower yoke laminations 112 and 114. The second layermay be obtained by rotating the first or basic layer 180 about axis 116,as illustrated by arrow 118, with axis 116 being disposed longitudinallythrough the inner leg lamination 106 in the plane of the lamination.This arrangement crosses or places the mitered joints of the first andsecond layers-at substantially right angles to one another, both aboveand below the inner leg laminations. Or, the basic layer may be rotated180 about axis 124, as indicated by arrow 126, which axis is disposedperpendicular to the plane of the inner leg lamination 106. If thesecond layer is in rotational symmetry with the first layer about axis116, the third layer will be in rotational symmetry with the first layerabout axis 120, as indicated by arrow 122, with axis 120 beingperpendicular to the leg laminations, in the plane of the leglaminations, and the fourth layer will be in rotational symmetry withthe first layer about axis 124. If the second layer is in rotationalsymmetry with the first layer about axis 124, the third layer will be inrotational symmetry with the first layer about axis 120, and the fourthlayer will be in rotational symmetry with the first layer about axis116.

The yoke laminations for magnetic core 100 may be cut from a strip ofmagnetic material as illustrated in FIG. 8 for the yoke laminations ofmagnetic core 22, while the leg laminations may be cut from a strip ofmagnetic material as illustrated in FIG. 10. FIG. 10 illustrates a strip130 of magnetic material being advanced in the direction of arrow 132,cutting outer leg lamination 102, inner leg lamination 106, outer leglamination 104, outer leg lamination 102' for the next layer, inner leglamination 106 for the next layer, and outer leg lamination 104 for thenext layer. The incremental narrowing of one end of each of the innerleg laminations 106 and 106' is indicated by dotted lines 108 and 108',respectively, with the narrowing occuring on the leading edge of theselaminations.

In summary, there has been disclosed new and improved electricalinductive apparatus, such as transformers or reactors, which have a newand improved magnetic core structure which has the advantages of the Xjoint above and below the inner leg laminations of a three-leggedmagnetic core structure, such as ease of joint closure and the highstrength of such a joint, without the disadvantages of the jointrelative to magnetic performance of the core. The new and improvedmagnetic core structure has only five different lamination shapes perlayer, and only one basic layer construction is utilized. The basiclayer is placed into four different orientations, to form a basic groupof laminations in which all of the joints are distributed into differentplanes. Therefore, three layers of laminations separate repeating jointsin the same plane, to substantially improve the magnetic performance ofthe core, compared to magnetic cores which utilize square joints withonly one layer of laminations between repeating joints in the sameplane. Further, the disclosed magnetic core may be formed in asubstantially scrapless manner, with the only scrap being generated bythe incremental narrowing of one end of each of the inner leglaminations. The incremental narrowing is necessary in order todistribute all the joints above and below the inner leg laminations intodifferent planes in each basic group of laminations.

Since numerous changes may be made in the abovedescribed apparatus anddifferent embodiments of the invention may be made without departingfrom the spirit thereof, it

is intended that all matter contained in the foregoing description orshown in the accompanying drawings shall be interpreted as illustrative,and not in a limiting sense.

Iclaim:

l. A magnetic core comprising:

at least one group of four stacked layers of metallic laminations;

a first layer of said at least one group including first and secondouter leg laminations, an inner leg lamination, first and second upperyoke laminations which join said outer leg laminations with miteredjoints, and said inner leg lamination with mitered and square joints,respectively, and first and second lower yoke laminations which joinsaid outer leg laminations with mitered joints and said inner leglamination with mitered and square joints, respectively; and

one end of said inner leg lamination having a step which narrows thewidth of the lamination where the inner leg lamination joins a yokelamination with a square joint; each lamination of said first layerhaving a duplicate in each of the three removing layers of the group,with the laminations of the three remaining layers being assembled thesame as the first layer, each of said layers being oriented todistribute each joint into different planes through the group.

2. The magnetic core of claim 1 wherein each of the threeremaininglayers of the group is in l80 rotational symmetry with the first layer,each about a different axis of the first layer.

3. The magnetic core of claim 2 wherein the axes of the first layerabout which the three-remaining layers are in rotational symmetry withare:

a. an axis which extends longitudinally through the inner leglamination, in the plane of the lamination;

b. an axis which extends perpendicularly through the plane of the innerleg lamination; and

c. an axis which is perpendicular to the sides of the inner leglamination, in the plane ofthe lamination.

4. The magnetic core of claim 3 wherein the inner leg lamination hassubstantially the configuration of a parallelogram, and the second,third and fourth layers are oriented relative to the first layer in thesequence a, b, and c.

5. The magnetic core of claim 3 wherein the inner leg lamination hassubstantially the configuration of a parallelogram, and the second,third and fourth layers are oriented, relative to the first layer, inthe sequence c, b and a.

6. The magnetic core of claim 3 wherein the inner leg lamination hassubstantially the configuration of a trapezoid, and the second, thirdand fourth layers are oriented, relative to the first layer, in thesequence a, c and b.

7. The magnetic core of claim 3 wherein the inner leg lamination hassubstantially the configuration of a trapezoid, and the second, thirdand fourth layers are oriented, relative to the first layer, in thesequence b, c and a.

8. The magnetic core of claim 1 wherein the first upper and first loweryoke laminations have substantially the same configuration anddimensions, and the second upper and second lower yoke laminations havesubstantially the same configuration and dimensions.

9. The magnetic core of claim 8 wherein the innner leg lamination hassubstantially the configuration of a parallelogram, with the first upperand first lower yoke laminations joining the first and second outer leglaminations, respectively, and the second upper and second lower yokelaminations joining the second and first outer leg laminations,respectively.

10. The magnetic core of claim 8 wherein the inner leg lamination hassubstantially the configuration of a trapezoid, with the first upper andfirst lower yoke laminations joining the first outer leg lamination, andthe second upper and second lower yoke laminations joining the secondouter leg lamination.

11. The magnetic core of claim 1 including a plurality of groups of fourlayers of laminations, stacked in superposed relation, each constructedsimilar to the at least one group of laminations.

1. A magnetic core comprising: at least one group of four stacked layersof metallic laminations; a first layer of said at least one groupincluding first and second outer leg laminations, an inner leglamination, first and second upper yoke laminations which join saidouter leg laminations with mitered joints, and said inner leg laminationwith mitered and square joints, respectively, and first and second loweryoke laminations which join said outer leg laminations with miteredjoints and said inner leg lamination with mitered and square joints,respectively; and one end of said inner leg lamination having a stepwhich narrows the width of the lamination where the inner leg laminationjoins a yoke lamination with a square joint; each lamination of saidfirst layer having a duplicate in each of the three removing layers ofthe group, with the laminations of the three remaining layers beingassembled the same as the first layer, each of said layers beingoriented to distribute each joint into different planes through thegroup.
 2. The magnetic core of claim 1 wherein each of thethree-remaining layers of the group is in 180* rotational symmetry withthe first layer, each about a different axis of the first layer.
 3. Themagnetic core of claim 2 wherein the axes of the firSt layer about whichthe three-remaining layers are in rotational symmetry with are: a. anaxis which extends longitudinally through the inner leg lamination, inthe plane of the lamination; b. an axis which extends perpendicularlythrough the plane of the inner leg lamination; and c. an axis which isperpendicular to the sides of the inner leg lamination, in the plane ofthe lamination.
 4. The magnetic core of claim 3 wherein the inner leglamination has substantially the configuration of a parallelogram, andthe second, third and fourth layers are oriented relative to the firstlayer in the sequence a, b, and c.
 5. The magnetic core of claim 3wherein the inner leg lamination has substantially the configuration ofa parallelogram, and the second, third and fourth layers are oriented,relative to the first layer, in the sequence c, b and a.
 6. The magneticcore of claim 3 wherein the inner leg lamination has substantially theconfiguration of a trapezoid, and the second, third and fourth layersare oriented, relative to the first layer, in the sequence a, c and b.7. The magnetic core of claim 3 wherein the inner leg lamination hassubstantially the configuration of a trapezoid, and the second, thirdand fourth layers are oriented, relative to the first layer, in thesequence b, c and a.
 8. The magnetic core of claim 1 wherein the firstupper and first lower yoke laminations have substantially the sameconfiguration and dimensions, and the second upper and second lower yokelaminations have substantially the same configuration and dimensions. 9.The magnetic core of claim 8 wherein the innner leg lamination hassubstantially the configuration of a parallelogram, with the first upperand first lower yoke laminations joining the first and second outer leglaminations, respectively, and the second upper and second lower yokelaminations joining the second and first outer leg laminations,respectively.
 10. The magnetic core of claim 8 wherein the inner leglamination has substantially the configuration of a trapezoid, with thefirst upper and first lower yoke laminations joining the first outer leglamination, and the second upper and second lower yoke laminationsjoining the second outer leg lamination.
 11. The magnetic core of claim1 including a plurality of groups of four layers of laminations, stackedin superposed relation, each constructed similar to the at least onegroup of laminations.